Electron beam apparatus

ABSTRACT

A high energy electron beam apparatus is provided having a cathode assembly for operation within a high temperature environment. The assembly associated with the cathode is subjected to high temperature generated by the cathode and associated excitation source. More specifically, this invention is directed to the support assembly associated with and about the cathode which includes sealed conductors with insulation means provided thereabout and also means for supporting and positioning a bolt type cathode accurately with respect to the assembly.

United States Patent 11 1 Fink [ 1 Apr. 10, 1973 I 1 ELECTRON BEAM APPARATUS [75] Inventor: Joel H. Fink, Pittsburgh, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Jan. 20, 1971 21 App1.No.: 108,039

[52] US. Cl. ..313/237, 313/278, 313/305,

313/310, 313/336, 313/337, 313 347 51 1111.0 ..H0lj1/94,H0lj 1/20,1101i H16 [58] Field of Search ..313/236, 237, 278, 313/302, 310, 347, 336, 337, 305

[56] References Cited UNITED STATES rATENTs 2,509,053 5/1950 Calbick ..3l3/336 X 2,455,381 12/1948 Morton et al... ....3l3/337 X 3,462,635 8/1969 Broers ....313/337 X 3,268,305 8/1966 Hagadom et a1. "313/337 X 1,794,950 3/1931 Freeman ..3 13/310 X 2,602,907 7/1952 Shower ..3 1 3/278 2,208,406 7/1940 Benedict.... .....3l3/237 X 1,520,674 12/1924 Eldridge ..3 13/236 X Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher Attorney-1 H. Henson and C. F. Renz 57 ABSTRACT A high energy electron beam apparatus is provided having a cathode assembly for operation within a high temperature environment. The assembly associated with the cathode is subjected to high temperature generated by the cathode and associated excitation source. More specifically, this invention is directed to the support assembly associated with and about the cathode which includes sealed conductors with insulation means provided thereabout and also means for supporting and positioning a bolt type cathode accurately with respect to the assembly.

3 Claims, 5 Drawing Figures P/IIIiI-IIIZI] APR I 01973 sum 1 OF 3 MECHANICAL PUMP MECHANICAL PUMP MECHANICAL PUMP PROTECTIVE GAS FIG. 5

FIG.

1 ELECTRON BEAM APPARATUS BACKGROUND OF THE INVENTION Present day requirements of electron beam welding apparatus require a cathode of a suitable material which can function when exposed to a poor vacuum of greater than Torr. This means that one must resort to cathode materials which may not be the most efficient from the emission and temperature standpoint,

but will operate satisfactorily under these conditions. Such cathodes must operate at high temperatures to minimize emission poisoning effects of the residual gases. For example, a tungsten bolt type cathode may in the design of the support assembly. The insulating" material provided between conductors must be relatively free from evaporation or sublirn ation of materials from the structure so as to limit the surface leakage.

The insulating material utilized must be capable of withstanding this high temperature without breakdown or any substantial reduction in insulating properties. The components must not warp as a result of the high temperature gradients, because such distortions will modify the characteristics of the device or destroy its operability. I t In. prior art type devices, the bolt cathode has-been mounted within a sleeve-like member-and held in position by a set screw bearing on the side of the cathode. Another attempted solution to the mounting of the bolt cathode is to utilize a pin vice arrangement. One problem found with these two arrangements was that a welding effect caused sticking and that made it difi'icult to modify the position of the cathode after some operation. It was also found with the set screw configuration that on occasion the screw might become loose within the threaded hole or sometimes might bind after a small amount of operation. i

' With regard .to the assembly of the'conductive leads into the header by the standard technique of brazing, it was found that the low temperature braze material would sublime during operation, thereby causing surface leakage across the insulating members.

SUMMARY OF THE INVENTION In accordance with the present invention, the cathode assembly comprises a plurality of conducting members passing through a header and mounted therein by means ofan insulating sleeve provided about the ocnductor and a glass frit material providedbetween the insulating sleeve and the conductor and between the insulating sleeve and the header. In addition, the bolt type cathode is mounted in such a manner as to be spring loaded.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention,

reference may be had to' the preferred embodiment, ex-

emplary of the invention, shown in the accompanying drawings, in which:

' FIG. 1 is a schematic showing of an electron beam welding apparatus designed for operation in the atmosphere incorporating the teachings of this invention;

FIG. 2 is an enlarged sectional view illustrating the cathode and main supporting column and associated insulator;

FIG. 3 is a top view of a cathode assembly;

FIG. 4 is a sectional view taken along lines TV-IV of FIG. 3; and

FIG. 5 is an enlarged sectional view of the conducting lead'in FIG. 4 and associated support structure.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring in detail to FIG. 1, an electron beam apparatus is illustrated comprising three sections 10, 12 and 14. The top section or chamber 10 is a high pressure section which is filled with a suitable insulating gas such as SF and where the high voltage supplies are located. The middle section 12 is the section of the apparatus where the lowest vacuum is found. A cathode assembly 16 is mounted in the middle section 12. The succeeding or lower chamber 14 is where the electron Y beam generated by the cathode assembly 16 is directed and passes through succeeding pumping stages and finally to the'bottom orifice 18 where it goes on into the full atmosphere and bombards a workpiece 20.

Several pumping stages and several pumping orifices are shown in FIG. 1. The pressure increases stage by stage from the cathode assembly 16 on out to the worka Torr, and the final stage is at about 200 Torr. A magnetic lens 24 is utilized for focusing the electron beam.

The electron beam generated at the cathode assembly 16 in the low pressure region is accelerated by an anode member 17. The anode member 17 is at substantially ground potential. The cathode assembly 16 is at a negative potential of 150,000 volts while the succeeding stages of the system are all actually at about ground potential. The electron beam proceeds through an aperture in the anode member 17 and is focused by the magnetic lens 24 and passes on through several orifices separating differential pressures which are maintained by the various pumps located progressively along a tubular housing 26. The housing 26 maybe of a suitable metal such as non-magnetic stainless steel.

In FIG. 2, the cathode assembly 16 is shown mounted within a socket portion 30. The socket portion 30 is mounted on one end of a column assembly 32 that is supported at the other end by a conical insulator 34. The conical insulator 34 is made of a suitable material such as alumina. The column assembly 32 may be of suitable material such as a Kovar alloy. The conical insulator 34 provides insulation and mechanical support for the cathode assembly 16 within the system. The cathode assembly 16 plugs ito the socket 30 at the bottom of the column 32 and is held in place by a retaining member 36 which has an aperture 38 and forms a part of the electron optics.

The cathode assembly 16 is shown in detail in FIGS. 3 and 4. The assembly 16 comprises a cathode housing member 41 which includes cup shaped housing portions 43 and 45. The portion 43 includes a transverse portion 47 and most of the cathode assembly is attached to the portion 47 by suitable means such as screws.

An insulating ring assembly 49 is positioned within the housing portion 43 and is secured to the transverse portion 47. The ring assembly 49 includes a ring 51 of a suitable material such as a Kovar alloy. A plurality of electrical conductive support leads 46 are supported in apertures 53 positioned about the ring 51.

FIG. 5 shows in detail the mounting of one of the support leads 46 in the aperture 53. A suitable insulating sleeve 55 surrounds the support lead 46. A glass frit seal 57 secures the sleeve 55 to the support lead 46 and a glass frit seal 59 secures the sleeve 55 within the aperture 53 of the ring 51. The support leads 46 extend through apertures in the transverse member 47.

The bolt cathode 40 is mounted in a housing 61 which is made predominantly of a suitable metal such as Kovar. The housing 61 is secured to the transverse member 47 with an insulating disc 63 mounted therebetween. The bolt cathode 40 projects beyond the I housing 61 and through an aperture in the transverse member 47 The extended and free end of the bolt cathode 40 is surrounded by a heater member 42. The bolt cathode 40 is made of suitable refractory material such as tungsten and has a diameter of about 0.060 inch and about 2 inches long. The heater 42 emits electrons which bombard the bolt 40 thus providing the bolt 40 with sufficient energy to heat it up to electron emitting temperature about 2,500C. A tubular heat shield member 44 surrounds the heater 42 and provides better thermal efficiency. One leg of the heater 42 is supported by the heat shield 44 which in turn is supported by three of the support leads 46. The other leg of the heater 42 is supported by a member 79 which is in turn supported by an insulated lead 46. Above the heat shield 44 is a cap member 48 which is designed to reflect the energy back into the bolt 40 and make the system more thermally efficient.

The bolt cathode 40 is provided at one end with a lock washer 50 which is retained in a suitable notch.52 provided in the bolt 40. The retaining washer 50 is confined in a recess portion 54 of a bearing assembly 56. The bearing assembly 56 is comprised of a fixed sleeve member 58, a movable sleeve member 60 and a spring member 62. The bearing assembly 56 is positioned within a hollow portion 64 of the housing 61. The fixed member 58 of graphite is seated in the hollow portion 64. The movable member 60 is also of graphite and the spring member 62 applies pressure on the movable member 60 so that the bolt 40 is pressed against a bearing surface 66. The bolt 40 is located by two holes, one in the movable member 60 where the lockingwasher 50 is held and the other in the fixed member 58. The position of the bolt 40 is determined by the position of an adjustable bearing surface 66. The bearing surface or stop 66 is the end of a set screw 68 which thus allows adjustment of the position of the bolt 40. The lock washer 50 may be of any suitable metal with the relatively low coefficient of expansion such as a Kovar alloy. The seating of the lock washer 50 in the graphite cylindrical piece 64 is such that the fit of the washer 50 in the graphite member 64 determines the maximum allowable coefficient expansion of the washer 50. In some designs, the fit of the washer 50 in the receiving space in the graphite member 64 has confined the washer to the groove in the bolt thereby holding the bolt in place.

The set screw 68 which positions the bolt 40 is not too critical in terms of the coefficient expansion because of its short length and because it fits in a relatively massive metal housing 61 for which the temperature rise is not excessive. The two graphite members 58 and 60 that support the bolt 40 provide two positions for bolt location. The design is such that if the back cover 65 of the housing 61 is removed, the bolt 40 can be withdrawn from the back of the assembly without disturbing the heater structure in the front. The choice of graphite material for the bearing assembly 56 is important in that it provides an assembly which does not stick in the housing 61, nor to which the bolt 40 will stick after repeated use. Hence the bolt 40, or if need be the bolt 40 and the bearing assembly 56 may be removed and if necessary replaced in subsequent operations.

In operation, the bolt cathode 40 is provided with a potential of a negative 150,000 volts by a high voltage supply 69 in the chamber 10. The electron beam is generated and maintained by system of electron optics so that the position of the emitting face of the bolt cathode 40 relative to the other members of the structure is critical. The location of the bolt cathode 40 can greatly alter the shape of the beam. For this reason, the location of emitting face of the bolt cathode 40 is important and the set screw 68 in the back provides a precise location. After initial assembly the height of the cathode with respect to the cap is checked and measured and held within a half a thousand of an inch.

The heat shields 44 and 48 and heater 42 are supported by the support lead 46 which are in turn mounted in a low coefficient of expansion metal ring 51. The insulation between the lead 46 and the ring 51 is provided by means of the ceramic cylinder 55. The cylinder 55 may be of alumina of high purity or of any material which has the property having a high electrical 'resistance at a relatively high temperature. A solder glass material 57 is provided between the lead 46 and the sleeve 55 and solder glass material 59 is provided between the sleeve 55 and the ring member 51. The solder glass may be of any suitable type such as Pyroceram No. 7574 and may be purchased from Corning Glass Company of Coming, New York. In this fashion, the assembly is made by means of the appropriate gluing material 57 and 59 while the insulation is provided by the sleeve 55. This structure is assembled by means of the solder glass 57 and 59 and thereby held in place. The insulation is provided by means of the alumina sleeve 55 so that excellent high temperature properties of insulation are maintained. The surface leakage is minimized by extending che insulator 55 above and below the ring member 51 and by suitable shielding provided by the surrounding members. The use of devitrifying solder glass as the glueing materials 57 and 59 permits the fabrication of such an assembly from aprts which need not be made with any great precision. This comes about by virtue of the fact that the devitrifying glass solidifies while still hot. The fit between the alumina sleeve 55 and the metal members comprising the lead 46 and the ring 51 need not be accurate because the solder glass fills the voids. The glass is in the form of a suitable devitrifying solder glass such as Pyroceram No. 7574 mixed with nitro-cellulose and butyl-cellosolve in a suspension of catsup-like consistency that is painted upon the lead 46. Likewise, the frit is painted on the metal portion of the ring 51 that is going to be in contact with the ceramic sleeve 55, The three pieces 46, 55 and 51 are put together in a suitable jig which must be made of a low expansion metal. The assembly is then fired at approximately 800C for about 1 hour. In the course of this time, the crystal growth takes place in the devitrifying solder glass and the voids between the ceramic sleeve SSand the metals 46 and 51 are filled resulting in a strong assembly at room temperature. The lead 46 has a diameter of about 0.080 inch andis made of a low coefficient expansion material of electrical conducting such as a Kovar alloy.

I claim as my invention:

l. A high current density electron beam source commeans is of a high temperature spring material and secures saidbolt cathode against said adjustable stop. 

1. A high current density electron beam source comprising an elongated rod-like high temperature cathode of the bolt type for emission of electrons from one end with means distributed around said cathode and spaced therefrom for generating heat within the bolt cathode, spring means for positioning said bolt cathode longitudinally and an adjustable stop means bearing against the opposite end of said cathode with respect to the end from which electrons are emitted.
 2. The source set forth in claim 1 in which graphite bearings are positioned along the length of said bolt cathode.
 3. The soruce set forTh in claim 2 in which the spring means is of a high temperature spring material and secures said bolt cathode against said adjustable stop. 